For optical heads for optical disks, it is important to ensure the accuracy of tracking control for allowing an optical spot to accurately follow the center of an information track. If this control is inaccurate, then during recording, a signal on an adjacent information track may be erased or cross talk may increase. Consequently, critical malfunction may occur, such as a failure to accurately reproduce information.
A three-beam tracking method has been commonly known as a tracking error signal detecting method. An optical head based on this method forms not only a main beam used for recording on or reproduction from information tracks but also two other subbeams. Accordingly, the optical head separately receives reflected beams from respective optical spots condensed on an optical disk.
FIG. 38 is a diagram illustrating optical spots on an optical disk formed by an optical head based on the three-beam tracking method. Reference numerals 100 and 102 denote optical spots of two subbeams. Reference numeral 101 denotes an optical spot of a main beam. Reference numeral 103 denotes information tracks. The optical spots 100 and 102 of the subbeams are each formed at an equal distance, e.g. a ¼ track pitch from the optical spot 101 of the main beam in a direction perpendicular to the information tracks 103.
A reflected beam from each optical spot has its intensity modulated by the information tracks. Detected signals of the two subbeams have phases shifted from that of a signal of the main signal by the ¼ track pitch in the opposite directions. The optical head based on the three-beam tracking method is constituted to detect a tracking error signal from a difference between the signals of these two subbeams.
Further, as shown in the document “Optical Disk Technologies” (Radio Technology Inc.; issued on Feb. 10, 1989; pp. 93 to 96), tracking error signal detecting methods called a “composite continuous tracking method”, a “composite wobbled tracking method”, and a “sample servo tracking method” are also known.
With these methods, an optical disk has zigzag marks formed on information tracks using pits or the like or a mirror surface portion formed thereon. With the first and second conventional methods, an offset that may occur in a tracking error signal based on what is called a push-pull method is corrected by using a signal detected in the zigzag marks or mirror surface portion. With the third conventional method, a tracking error signal is detected by using the zigzag marks. The entire disclosure of the document “Optical Disk Technologies” (Radio Technology Inc.; issued on Feb. 10, 1989) is incorporated herein by reference in its entirety.
First, the first conventional method, the composite continuous tracking method will be described in detail with reference to drawings. FIG. 39 is a diagram illustrating the arrangement of a mirror surface portion formed on information tracks on an optical disk. Reference numeral 104 denotes information areas in which are formed pits and guide grooves having addresses, information, or the like recorded therein. Reference numeral 105 denotes a mirror surface portion formed between a series of information areas. Reference numeral 106 denotes a centerline of each information track.
For such an optical disk, an optical head uses an objective lens to condense light to form an optical spot, and receives a reflected beam from the optical spot, the beam being received so as to be divided into two parts along a parting line parallel with the information tracks. Then, the optical head detects a tracking error signal from a difference between the detected received light signals. Since the tracking error signal is based on the push-pull method, a deviation in the optical axis of the objective lens or the like may cause an offset in the tracking error signal. However, this offset has an amount corresponding to a value for the tracking error signal measured when the optical spot passes over the mirror surface portion. The optical head detects the tracking error signal when the optical spot passes over the guide groove, and detects the offset when the optical spot passes over the mirror surface portion. The optical head thus corrects the offset in the tracking error signal.
Now, the second and third conventional methods will be described in further detail with reference to drawings. FIG. 40 is a diagram illustrating the arrangement of zigzag marks. Reference numeral 107 denotes a first pit, and reference numeral 108 denotes a second pit. This pair constitutes the zigzag marks. These marks are arranged at predetermined distances from the tangential direction of the information track and, in a direction perpendicular to the information track, at an equal distance from the center of the information track in the opposite directions. Reference numeral 109 denotes an information area in which an address, information, or the like is recorded. Reference numeral 110 denotes a centerline of the information track. An arrow 111 indicates a scanning path 1 of the optical spot. An arrow 112 indicates a scanning path 2 of the optical spot.
FIG. 41 is a diagram illustrating signal waveforms indicative of the quantity of reflected light detected when the optical spot passes over the zigzag marks. Reference numeral 113, 114, and 115 denote signal waveforms obtained on the scanning path 1, the scanning path 2, and a scanning path extending along the centerline of the information track, respectively. Points on the axis of abscissas, denoted by positions A and B, represent the positions of the first and second pits, respectively. Symbols VA and VB in the drawing indicate values for signals sampled and held at these positions.
These signal values are determined by the relative positions of the optical spot and zigzag marks and are not substantially affected by a deviation in the optical axis of the objective lens. The optical head based on the second conventional method detects a tracking error signal on the basis of the push-pull method, compares a difference between the signals VA and VB with the tracking error signal to detect an offset in the tracking error signal, and then corrects the offset. Further, the optical head based on the third conventional method detects a tracking error signal on the basis of a difference between the signals VA and VB.
Next, description will be given of an optical head having optical disk tilt detecting means according to the prior art. When the optical head records or reproduces information on or from the optical disk, the optical axis of the objective lens is desirably at 90° to a surface of the optical disk. If the optical disk is tilted, aberration such as coma aberration may occur to degrade an optical spot condensed on the optical disk. A tilt in the optical disk has been commonly detected by an exclusive optical disk tilt detector provided in the optical head. However, the reduced size of the optical head makes it difficult to obtain a space in which the exclusive detector can be provided. Thus, a configuration has been proposed which incorporates the optical disk tilt detecting means in an optical system of the optical head.
By way of example, the conventional technique disclosed in Japanese Patent Laid-Open Publication No. 7-141673 will be described. The entire disclosure of Japanese Patent Laid-Open Publication No. 7-141673 is incorporated herein by reference in its entirety.
FIG. 42 is a diagram showing a configuration of an optical disk tilt detecting means according to the prior art. Reference numerals 1101, 1102,1103, and 1104 denote an optical disk, an objective lens, a light receiving lens, and an element that branches light, respectively. Reference numerals 1104a and 1104b denote micro prisms. Reference numerals 1105a, 1105b, 1106a, and 1106b denote light receiving elements. Reference numerals 1107 and 1108 denote addition amplifiers. Reference numeral 1109 denotes a differential amplifier.
The optical disk tilt detecting means constructed as described above operates as follows: A beam reflected by the optical disk 1101 passes through the objective lens 1102 and light receiving lens 1103 and then impinges on the element 1104 branching light. Those parts of the beam impinging on the element 1104 which are incident on two very small areas in which the micro prisms 1104a and 1104b are formed are polarized toward and received by the light receiving elements 1106a and 1106b, respectively. A beam impinging the areas other than the above described two very small areas is transmitted through the element and impinges on the light receiving elements 1105a and 1105b. That is, the beam is divided into two parts, which are then received by the respective elements. Signals detected by the respective light receiving elements are calculated by the addition amplifiers 1107 and 1108 and differential amplifier 1109, thereby detecting a tilt Trad in the optical disk.
The signal detected by each light receiving element is substituted with the name of the light receiving element, the tilt Trad in the optical disk is detected using a calculation expressed by:Trad=1105a−1105b−(1106a−1106b)  (Equation 1)
FIG. 43 is a diagram illustrating the element 1104 branching light. A circle 1110 inside a rectangle indicating the element 1104 represents the shape of an incident beam. The micro prisms 1104a and 1104b are very small areas arranged laterally (in a direction perpendicular to the information tracks) symmetrically with respect to the center of the beam as shown in the drawing. Further, the dotted lines in the drawing show how the beam is guided to the light receiving elements 1105a, 1105b, 1106a, and 1106b. The beam incident on the light receiving elements 1105a and 1105b is divided into two parts at the boundary line between the elements in a direction parallel with the information tracks. Accordingly, detection of the optical disk tilt Trad based on Equation 1 corresponds to comparison of the magnitude of push-pull signals detected in the two very small areas with the magnitude of push-pull signals detected in areas other than the above very small areas.
FIGS. 44(a) to 44(c) are diagrams illustrating the distribution of light intensity of a beam incident on the element 1104 branching light. FIG. 44(a) indicates the case in which the optical disk is not tilted. FIGS. 44(b) and 44(c) indicate the case in which the optical disk is tilted in its radial direction (tilted so that a plane containing a normal of the optical disk and the optical axis of the objective lens is perpendicular to a tangent of the information tracks). For the direction of a tilt, FIG. 44(b) shows a positive direction, whereas FIG. 44(c) shows a negative direction. The shaded parts in FIG. 44(a) indicate the areas in which a positive and negative first-order diffracted beams, diffracted by the information tracks of the optical disk, are superimposed on a zero-order diffracted beam. In these areas, when the wavefront phases of the zero- and first-order diffracted beams change, interference may occur to change light intensity.
Further, the shape of the laterally asymmetric shaded parts shown in FIGS. 44(b) and 44(c) indicates the asymmetry of the distribution of light intensity. This indicates that a change in wavefront phase caused by coma aberration resulting from a tilt in the optical disk is laterally asymmetric with respect to the direction of the information tracks, so that the interference between the zero-order diffracted beam and the positive and negative first-order diffracted beams changes light intensity. Consequently, a darker and lighter parts appear in the asymmetric distribution of light intensity depending on the direction of a tilt in the optical disk.
The prior art pays attention to the lateral asymmetry of light intensity distribution, which depends on a tilt in the disk. Then, the optical disk tilt Trad is detected by comparing the magnitude of push-pull signals (1106a-1106b) detected by extracting light from the two very small areas, in which the asymmetry is most pronounced, with the magnitude of push-pull signals (1105a-1105b) detected in light from the other areas.
As described above, the conventional tracking error signal detecting methods are classified into two groups. One of the groups detects a tracking error signal by utilizing the fact that the quantity of light reflected from an optical spot varies with the relative positions of the optical spot and information tracks or marks formed on the optical disk (three-beam tracking method and sample servo tracking method). The other group uses the push-pull method to detect a tracking error signal, while detecting an offset in the tracking error signal to correct the signal itself (composite continuous tracking method and composite wobbled tracking method). These two types of detecting methods all have the advantage of reducing a possible offset in the tracking error signal, caused by a deviation in the optical axis of the objective lens.
However, when the optical disk is tilted from the optical head in its radial direction, coma aberration occurs to change the relative positions of the optical spot and information tracks, thereby changing the quantity of light reflected from the optical spot or the push-pull signal to cause a phase shift. As a result, a similar phase shift occurs in the tracking error signal. Therefore, with the above conventional tracking error detecting methods, if the optical disk is tilted, tracking control may disadvantageously be provided at a position separate from the center of the information track.
Further, the conventional techniques discussed in the latter half of the above description divide a beam into two parts along a parting line extending parallel with the information tracks to detect push-pull signals and thus a tilt in the optical disk. Thus, disadvantageously, a tilt in the optical disk cannot be accurately detected if the optical disk has information tracks in which push-pull signals cannot be properly detected, for example, information tracks having a groove depth equal to a ¼ wavelength.
Further, the above very small area is smaller than the area in which a zero-order diffracted beam and a positive and negative first-order diffracted beams diffracted by the optical disk are superimposed on one another. Thus, when the objective lens is moved in a direction perpendicular to the information tracks, the position of a detected beam is also moved. Consequently, light having that part of a light intensity distribution which does not accurately reflect a tilt in the optical disk is incident on the very small area. As a result, a tilt in the optical disk is less accurately detected.